This invention relates to an improved continuous-feed process for the liquefaction of coal (anthracite, bituminous, sub-bituminous), gob, bitumen, lignite, oil and tar sands, oil shale, and any solid carbonaceous material, including waste material and plastic material and for the distillation of the volatile matter within that solid carbonaceous material into high-value products.
Until recent decades despite voluminous amounts of CO2 emissions and other contaminants, coal providers have experienced very little ecological pressure from governments. While coal is cheap and produces significant quantities of power, it is also an international “necessity” because the world could not immediately replace this energy source. However, as the world has focused on environmental efficacy, better systems and methods of using the energy stored in coal become more important.
All coal contains varying concentrations of moisture, sulfur, hydrocarbon compounds (referred to as volatile matter), inorganic ash-forming components, and other components. Some of these components have value while other components are considered contaminants. Synthetic production of liquid fuels (i.e., gasoline and oil substitutes) in the United States has a long history. In the 19th century, dozens of facilities produced oil, gas, grease and paraffin from coal, but by 1873, cheap petroleum caused the last coal oil plant to close. In addition, commercial scale shale oil extraction began in 1857 at shale oil retorts retorting the Devonian oil shale along the Ohio River Valley. However, after crude oil discovery in Pennsylvania in 1859, oil shale industries found it difficult to compete and they were shut down by 1861.
Historically, economics has been a major impediment to coal liquefaction. Until recent years oil has been easy to find and produce. In addition, a powerful liquid oil industry has lobbied and maintained a unique control over domestic oil production. The international landscape is now aware of the imminent danger of deep water drilling for oil as evidenced by the British Petroleum oil spill in the Gulf of Mexico in April of 2010.
There are several processes used for coal liquefaction. For example, in the Bergius process, developed by Friedrich Bergius in 1913, dry coal is mixed with heavy oil recycled from the process. A catalyst is typically added to the mixture. The reaction occurs at between 400° C. (752° F.) to 5,000° C. (9,030° F.) and 20 to 70 MPa hydrogen pressure.
Chevron Corporation developed a process that involved close-coupling of the non-catalytic dissolver and the catalytic hydroprocessing unit. The oil produced was lighter and had far fewer heteroatom impurities than other coal oils. Apparently, the process was scaled-up to the 6 ton per day level, but has not been proven commercially.
The Karrick process is a low-temperature carbonization (LTC) and pyrolysis process of carbonaceous materials. Although primarily meant for coal carbonization, it also could be used for processing of oil shale, lignite or other carbonaceous materials. These are heated at 450° C. (800° F.) to 700° C. (1,300° F.) in the absence of air to distill out synthetic fuels-unconventional oil and syngas. The Karrick process may be used for coal liquefaction and for semi-coke production.
In the Karrick process, one short ton of coal yields as much as one barrel of oils and coal tars (12% by weight), 3,000 cubic feet (85 cubic meters) of coal gas and 1,500 pounds (680 kg) of solid smokeless char or semi-coke (for one metric ton, the results would be 0.175 m3 of oils and coal tars, 95 m3 of gas, and 750 kg of semi-coke). Yields by volume of approximately 25% gasoline, 10% kerosene and 20% fuel oil are obtainable from coal. Gasoline obtained from coal by the Karrick process combined with cracking and refining is equal in quality to tetraethyl lead gasolines. More power is developed in internal combustion engines and an increase in fuel economy of approximately 20% is obtainable under identical operating conditions. The syngas can be converted to oil by the Fischer-Tropsch process. Coal gas from Karrick LTC yields greater energy content than natural gas.
Compared to the Bergius process, the Karrick process is cheaper, requires less water and destroys less thermal value (one-half that of the Bergius process). The smokeless semi-coke fuel, when burned in an open grate or in boilers, delivers 20% to 25% more heat than raw coal. The coal gas should deliver more heat than natural gas per heat unit contained due to the greater quantity of combined carbon and lower dilution of the combustion gases with water vapor.
The cheapest liquid fuel from coal will come when processed by LTC for both liquid fuels and electric power. As a tertiary product of the coal distilling process, electrical energy can be generated at a minimum equipment cost. A Karrick LTC plant with one kiloton of daily coal capacity produces sufficient steam to generate 100,000 kilowatt hours of electrical power at no extra cost excepting capital investment for electrical equipment and loss of steam temperature passing through turbines. The process steam cost could be low since this steam could be derived from off-peak boiler capacity or from turbines in central electric stations. Fuel for steam and superheating would subsequently be reduced in cost.
Although a Karrick pilot plant was successfully operated in 1935, there is some question as to whether a modern commercial Karrick LTC process plant would fail due to mechanical problems, a postulation based on previous failures of other plants using different processes under different conditions. It is indeterminate as to how “scaleable” the technology is for large-scale production. When oil was significantly cheaper markets for the described coal products were limited, which made such a venture economically unsound.
Other methods of coal liquefaction involve indirect conversion. Perhaps the main indirect process is the Fischer-Tropsch process, in which coal is first gasified to make syngas (a balanced purified mixture of CO and H2 gas). Next, Fischer-Tropsch catalysts are used to convert the syngas into light hydrocarbons (like ethane) that are further processed into gasoline and diesel. This method was used on a large technical scale in Germany between 1934 and 1945 and is currently being used by Sasol in South Africa. In addition to creating gasoline, syngas can be converted into methanol, which can be used as a fuel or a fuel additive. Syngas may be converted to liquids through conversion of the syngas to methanol, which is subsequently polymerized into alkanes over a zeolite catalyst.
Unfortunately, each of the prior methods of coal liquefaction have disadvantages. The prior processes tend to focus on turning coal to liquid, with little regard for environmental implications. For example, Fischer-Tropsch produces toxic byproducts and consumes expensive catalysts during the process (cobalt, iron, ruthenium). The prior processes have often not been scalable, and thus were of limited viability. Many also had significant capital costs that tended to render the liquefaction economically suspect.